First law of railfanning, there will always be plenty of action, except where you are.....

Via I-8 you hit the UP in Yuma which has the depot, the yard, the Yuma territorial prison (right next to the track and above the River for great train watching, and the Colorado River Bridge, with trains entering and leaving Yuma stopping to change crews. If you are going via Glamis (cut off diagonal from I-10 to El Centro and I-8 to San Diego) you take your chance with what is coming on the UP main, there at the dunes.

Further down I-8 is El Centro, and locals, Off of I-8, west of El Centro is the narrow guage US Gypsum operations at Seeley, and up the hill as you head into San Diego County, there is what is left in place, but not used, of the SDA&E line, including Carrizo Gorge, and the Pacific Southwest RR museum in Campo, which was the headquarters of the US black cavalry, before WWII.

Further down the road, when you arrive in downtown San Diego, you have everything, including dual use LRT and freight lines out to places like El Cajon, a big LRT system using many of the old freight branches, at San Diego Station, you have your choice of LOSSAN, Coaster, during the day, and BNSF related freight, only at night.

Also, in Balboa Park is the San Diego RR museum, with severl giant layouts, including the SDAE, and a massive layout featuring the SP from Bakersfield to Mojave, over the Tehachipi pass, in what seems like actual size.

If you go via I-10 and the IE, you have Beaumont pass, two tracked CTC UP (former SP) Sunset main with stack trains to an from POLA/POLB all day and night (just not always when you get there...) parallel all the way from Indio to Beaumont....